The present invention relates in general to detecting and correcting for multipath interference in an FM radio receiver, and more specifically, to faster, more accurate detection of the existence of multipath events and to more effective correction without adding other distortion or undesirable noises.
The problem of multipath distortion in radio receivers is well known. Multipath events occur when radio frequency (RF) signals following a direct path and one or more indirect (i.e., reflected) paths from a transmitter to a receiver interfere with each other at the receiver. Reflections can be caused by hills and buildings, for example.
Constructive and destructive interference of signals caused by interaction between the reflections and the direct line of sight transmission causes both signal distortion and rapid fluctuations in the received field intensity, especially in moving vehicles.
Multipath is a particularly annoying problem in reception of FM stereo broadcasts due to the greater bandwidths of FM stereo broadcasts, corruption of the stereo pilot carrier, and sensitivity to phase errors. Prior art systems respond to multipath conditions by adjusting stereo separation (i.e., blend) and/or adding a high frequency cutoff for the reproduced audio signal. In order to avoid having these measures become noticeable, they are gradually introduced and then gradually eliminated after the multipath conditions cease. Thus, these prior art measures degrade the overall audio signal fidelity even when there is no multipath disturbance present at a particular moment. Thus, prior systems tend to be slow to react at the onset of multipath so that corrections are not put in place in time to prevent multipath distortion from reaching the audio output. Furthermore, the decreased fidelity resulting from the correction measures continue to negatively impact reception at times when they are no longer necessary.